Remove dependencies on dataclasses

[cmungall]

When linkml-owl was written, python generated dataclasses were the standard way of working with linkml data. It is now more common to use pydantic (which is also much faster). The core linkml-runtime dumpers now accepted either, but linkml-owl lags behind.

And in fact, owl_dumper, doesn't even need objects, with some minor modifications it could work with dicts